The invention relates to a system for adjusting the tension of the continuous part of a continuously variable transmission.
Such a system is, for example, known from U.S. Pat. No. 5,337,628, In addition to the continuously variable transmission, which is adjusted continuously with respect to its transmission ratio, the. following are arranged in the drive train: a vehicle engine, a torque converter which is bridgeable by a bridging clutch and clutches for the switchover between forward travel and reverse travel. This publication too relates to the adjustment of the tension of a continuous part in a continuously adjustable transmission which comprises the continuous part and drive and output conical discs. The tension of the continuous part is adjusted by the hydraulic pressure in an output-end oil chamber.
The tension of the continuous part is to be so adjusted that the efficiency of the continuously variable transmission is a maximum. It is here, on the one hand, to be prevented that the continuous part slips because of a tension which is too low and, on the other hand, the tension of the continuous part should not be too high in order to avoid high losses in the continuously variable transmission. To bring both requirements into harmony, the torque, which is transmitted from the drive end to the output end, must be known as accurately as possible. The torque at the drive disc to be transmitted is determined primarily from the torque of the vehicle engine and the torque amplification factor of a torque converter which is built in in some circumstances. A consideration of the operating states of the clutch when adjusting the tension of the continuous part does not take place. The task of the present invention is the optimization of the adaptation of the belt tension to the actual requirements.
As already mentioned, the invention proceeds from a system for adjusting the tension of a continuous part of a continuously variable transmission which is continuously adjustable preferably with respect to its transmission ratio. The continuously variable transmission is, together with a vehicle engine, mounted on at least one clutch in the drive train of the vehicle. The clutch exhibits various operating states. The essence of the invention is that the adjustment of the tension of the continuous part takes place at least in dependence upon the operating state of the clutch. In this way, a very precise adaptation of the belt tension to the actually required torque transmission is ensured.
In an advantageous configuration of the invention, it is provided that the adjustment of the tension takes place with the adjustment of a hydraulic torque. The adjustment of the hydraulic torque therefore takes place in dependence upon the operating state of the clutch. Here, it is especially provided that the continuously variable transmission has a drive end and an output end which have essentially the form df pulley discs. At least one belt (preferably a thrust element belt or a belt or a chain) is tension mounted as the continuous part between disc pairs which define the drive end and the output end. By adjusting the hydraulic pressure, the contact pressure of at least one pulley disc against the continuous part is adjusted.
In one embodiment of the invention, a clutch is provided which is driven for engaging the forward gear and the reverse gear. In this context, a forward clutch and a reverse clutch are especially considered. In addition, a torque amplifying planetary set can be mounted between the vehicle engine and the continuously variable transmission in the drive train. The planetary set is switched into the drive train when the reverse gear is engaged.
Furthermore, a torque converter can be mounted in the drive train of the vehicle and a converter bridging clutch can be provided as a clutch. The torque converter can be bridged by closing the converter bridging clutch.
The clutches assume at least the open and the closed state as operating states. Preferably, an additional operating state is present during closing of the clutches.
In an especially advantageous embodiment of the invention, it is provided that the tension of the continuous part and/or the desired contact pressure are differently computed in each case in dependence upon the actuating state of the forward and reverse clutches. Here, it is preferably provided that the tension of the continuous part is increased or reduced in response to the closing of the reverse clutch. This has the background that, when engaging the reverse gear, the torque amplifying or torque reducing planetary set is switched into the drive train whereupon the contact pressure or the tension should be increased or reduced for avoiding an impermissible slip.
In another advantageous embodiment of the invention, it is provided that the adjustment of the tension takes place in dependence upon a determined torque quantity which represents the torque acting at the input of the continuously variable transmission. The determination of this torque quantity takes place in dependence upon the operating state of the clutch.
In a further embodiment of the invention, it is provided that a slip quantity, which represents the slip of the torque converter, and an engine torque quantity, which represents the engine output torque, are determined. For an opened converter bridging clutch, the torque quantity, which represents the transmission input torque, is determined by a first determination at least in dependence upon the determined slip quantity. For a closed converter bridging clutch, the torque quantity is determined via a second determination at least in dependence upon the determined engine torque quantity. This means that the desired contact pressure, and therefore the tension of the continuous part, is computed in dependence upon the actuating state of the converter bridging clutch.
For an open converter bridging clutch, the primary torque, that is, the transmission input torque, is computed from the engine rpm and the turbine rpm in correspondence to the physical equations of the torque converter. This computation method is more precise because the load signal, which is available from the vehicle engine, often contains large inaccuracies, for example, because of ancillary equipment not detected and because of friction in the vehicle engine. In systems, which use the engine torque directly for tension control, these inaccuracies lead to increased inaccuracies in the computation of the contact pressure or the tension (especially at standstill of the vehicle, that is, for high amplification of the torque converter). The contact pressure or the tension is, for this reason, too high in such systems when bringing the vehicle into motion from standstill whereby unnecessarily high power is converted in the transmission.
For a closed converter bridging clutch, the primary torque (that is, the transmission input torque) is computed from the engine characteristic field, corrected by the inertial torque of the engine and the torque take-up of the pump.
During closing of the converter bridging clutch, it is especially advantageous to select, as the torque quantity, the maximum value from the above-mentioned first and second determination. This means that, when closing the converter bridging clutch, a switchover is made smoothly between the two computing methods for the primary torque in that the larger computed primary torque is used.
In a further advantageous embodiment of the invention, it is provided that, during the closing of the clutches, the tension of the continuous part, and therefore the contact pressure, is increased compared to the tension or compared to the contact n pressure in the previous operating state. In this embodiment of the invention wherein the contact pressure or the tension is increased during the switching in of the clutches, possible torque impacts can be kept away from the continuous part. This can happen when switching in the forward or reverse clutches or when closing the converter bridging clutch.
In a further advantageous embodiment of the invention, it is provided that the adjustment of the tension of the continuous part takes place hydraulically and a hydraulic pump is provided which is driven by the vehicle engine. The engine torque quantity, which represents the engine output torque, is, in this embodiment, determined in dependence upon a pressure quantity representing the operating state of the pump. This means that, when computing the primary torque (especially for a closed converter clutch), the torque, which is taken up by the hydraulic pump, is considered.
The adjustment of the tension of the continuous part can furthermore take place in dependence upon whether an adjustment of the transmission ratio of the continuously variable transmission takes place toward larger or smaller ratios. Here, it is especially provided that the tension or the contact pressure during an adjustment to higher transmission ratios is increased compared to the adjustment of a transmission ratio which is essentially constant. In this way, it is possible in an advantageous manner to achieve more rapid transmission ratio adjustments to larger transmission ratios and to avoid belt slippage because of a primary pressure which is too low.
In a further advantageous embodiment, it is provided that the vehicle has an anti-blocking control system and/or a drive slip control system and/or a directional stability control system. The braking forces, which act on the wheels of the vehicle, are modified by means of these control systems in the active state thereof. The adjustment of the tension of the continuous part or the adjustment of the contact pressure takes place in this embodiment of the invention also in dependence upon the activation of such a control system. For the activation of such control systems, it is especially provided that the contact pressure and therefore the tension is increased in order to protect the continuous part against large torque impacts.